Hubble finds tiny moon near Pluto

This image provided by NASA combines two labeled images of the Pluto system taken by the Hubble Space Telescope's Wide Field Camera 3 ultraviolet visible instrument with a newly discovered fourth moon P4 circled. (AP)

The space telescope was searching for rings around the planetary oddball at the edge of our solar system when it came across P4, the temporary name for the newly discovered moon.

With an estimated diameter of 8 to 21 miles (13 to 34 km), P4 is the smallest of Pluto's four moons, the U.S. space agency said in a statement.

Pluto's largest moon, Charon, is 648 miles (1,043 km) across, and its other moons, Nix and Hydra, are in the range of 20 to 70 miles (32 to 113 km) in diameter.

"I find it remarkable that Hubble's cameras enabled us to see such a tiny object so clearly from a distance of more than 3 billion miles (5 billion km)," said Mark Showalter of the SETI Institute in Mountain View, California, who led this observing program with Hubble.

The observation by Hubble is part of ongoing work to support NASA's New Horizons mission, scheduled to have a close encounter with Pluto and its moons in 2015.

P4 is located between the orbits of Nix and Hydra, both of which were discovered by Hubble in 2005. Charon was discovered in 1978 at the U.S. Naval Observatory.

All four of Pluto's moons are believed to have formed when Pluto and another planet-sized body collided in the early history of our solar system. Earth's Moon may have formed the same way.

P4 was first seen in a photo taken by Hubble on June 28 and was confirmed in subsequent Hubble pictures taken July 3 and July 18, NASA said.

In June, Pluto came between a star and Earth, casting a small shadow on Earth's surface that astronomers tracked across the Pacific.

This event, known as an occultation, occurred on June 23, according to scientists at Lowell Observatory in Arizona. Four of them travelled on a modified 747 aircraft that carried a big telescope, which managed to snap images of Pluto and its thin atmosphere.

Learning more about Pluto's tenuous atmosphere is possible because the starlight behind it dims in a specific way, which lets astronomers determine atmospheric temperature and density, Lowell Observatory said in a statement on Wednesday.

Pluto cast an extremely long shadow: it has an average distance of 5.9 billion miles (9.495 billion km) from the Sun, compared to Earth's distance of 93 million miles (149.7 million km).